A number of cooled catheter systems have been developed for treating patients. Such systems may provide a variety of cryogenic and/or cooling treatment or diagnostic procedures, including cooling a desired tissue region, such as a portion of the heart, to stun it and allow cold mapping of the heart and/or confirmation of a catheter position with respect to localized tissue lesions, or to apply a more severe level of cold to ablate tissue at the site of the catheter ending.
In general, when used for endovascular access to treat the cardiac wall, catheters of this type, in common with the corresponding earlier-developed radio frequency or electrothermal ablation catheter, must meet fairly demanding limitations regarding their size, flexibility, and the factors of strength, electrical conductivity and the like which affect their safety and may give rise to failure modes in use. These constraints generally require that the catheter be no larger than several millimeters in diameter so as to pass through the vascular system of the patient to the heart. Thus, any electrodes (in the case of mapping or RF/electrothermal ablation catheters), and any coolant passages (in the case of cryocatheters) must fit within a catheter body of small size.
A number of different fluids have been used for the coolant component of prior art cryotreatment catheters, such as a concentrated saline solution or other liquid of suitably low freezing point and viscosity, and of suitably high thermal conductivity and heat capacity, or a liquified gas such as liquid nitrogen. In all such constructions, the coolant must circulate through the catheter, thus necessitating multiple passages leading to the cooling area of the tip from the catheter handle.
Furthermore, conditions of patient safety must be considered, raising numerous problems or design constraints for each particular system. Thus for example, a high pressure may be required to circulate sufficient coolant through the catheter body to its tip and back, and the overall design of a catheter must be such that fracture of the catheter wall or leakage of the coolant either does not occur, or if it occurs, is harmless. Further, for an endovascular catheter construction, the presence of the coolant and circulation system should not substantially impair the flexibility or maneuverability of the catheter tip and body.
To some extent these considerations have been addressed by using a phase change material as the cryogenic fluid, and arranging the catheter such that the phase change, e.g., from a liquid to a gas, occurs in the treatment portion of the catheter tip. Another possible approach is to employ a pressurized gas, and configure the catheter for cooling by expansion of the gas in the tip structure. However, owing to the small size that such a catheter is required to assume for vascular insertion, or the awkwardness of handling a cryogenic treatment probe generally, the design of a safe and effective coolant circulation system which nonetheless dependably provides sufficient cooling capacity at a remote tip remains a difficult goal.
Moreover, when specialized fluids or coolants are selected for use, they are often available only in large, industrial-size containers that can take up a significant amount of space in an operating room or electrophysiology lab. In addition, if the coolant or tank provides for a limited number of procedures before needing to be re-filled or replaced, a hospital or treatment center may need to store numerous containers of the selected fluid or coolant to treat a steady stream of patients. Such storage and/or frequent replacement of one coolant tank with another can greatly increase the cost and reduce the efficiency of maintaining and operating a particular medical cooling system and associated devices.
Accordingly, it would be desirable to provide a coolant system that can be effectively and efficiently integrated into or otherwise used with existing fluid source systems within a hospital or treatment center to avoid the undesired costs and efforts associated with the frequent re-filling or replacing of coolant sources in medical cooling systems.